Circadian rhythms and environmental lighting regulate a number of endocrine and behavioral functions. Dispersed chick pineal cells remain rhythmic and responsive to light in culture. Cyclic AMP is a key regulator of melatonin production by these cells. Light and L-type calcium channels act in part through cyclic AMP but the circadian pacemaker does not. Regulation of melatonin production occurs via induction of the enzyme serotonin N-acetyltransferase. Lability of this enzyme has prevented purification or gene identification, but progress has been made toward increasing its amount and stability.